Storage stability is a requirement for powder coating compositions. If the compositions lack storage stability, the powder is likely to agglomerate (such as by caking or lumping). Agglomeration of the particles of powder is extremely undesirable and leads to problems in application of the powder to a substrate.
Amorphous polymer resins used for powder coatings require a glass transition temperature (Tg) of higher than 40.degree. C. and preferably higher than 55.degree. C. so that the powders made from such resins are storage stable. Coatings resulting from such resins have adequate hardness but limited flexibility as a result of the Tg of the resin.
Semi-crystalline polyesters with Tg lower than 55.degree. C. are known for use in powder coatings or adhesives and are the subject of several patents. This permits the use of resins of much lower Tg than 55.degree. C. and yet crystallinity provides storage stability. These resins have lower melt viscosity at curing temperature than their amorphous counterparts. Coatings from such resins have good flow, good adhesion, and are smooth and flexible. Films rsulting from these resins, however, are usually soft as a result of their low Tg.
U.S. Pat. No. 4,352,924 describes semi-crystalline polyesters derived from dimethyl trans-1, 4-cyclohexanedicarboxylate (DMCD) and 1,4-butanediol, and their utility as resins for powder coatings.
U.S. Pat. Nos. 4,442,270 and 4,387,214 describe semi-crystalline copolyesters derived from terephthalic acid and 1,6-hexanediol for use in primers or overcoatings for automobiles.
U.S. Pat. No. 4,4,012,363 describes semi-crystalline polyesters derived from terephthalic acid and 1,4-butanediol, and its use as thermoplastic powder coatings.
Japanese Patent No. JP 190,919 describes a composite thermoplastic resin layer comprising (a) a crystalline polyester derived from terephthalic acid and ethylene glycol and (b) a copolyester of terephthalic acid, isophthalic acid and ethylene glycol and 1,4-butanediol. The layer is applied as a pre-formed laminate.
We are not aware of any prior art relating to blends of amorphous polyesters and semi-crystalline polyesters for use in thermosetting powder coatings which achieve novel film properties such as those described herein.